Optical fiber technology is well suited for communications applications because optical fibers have a wide transmission bandwidth and relatively low attenuation. However, optical fiber interfaces to electronic and optical networks are expensive to manufacture because of the difficulty associated with mounting laser transmitting and receiving devices onto substrates and aligning them with separately mounted optical fibers. For this reason, optical fiber technology has been widely implemented in long haul communication systems where the interfaces are relatively few. Long haul communications systems are also relatively price insensitive. However, the high cost of manufacturing fiber optic interfaces has been a barrier that has slowed the penetration of fiber optic technology into local metropolitan area communication systems and other markets.
To appreciate the coupling and alignment difficulties associated with coupling lasers to optical fibers, one must consider the geometry and dimensions of optical fibers, optical packages and laser transmitting and receiving devices. An optical fiber is an extruded, typically glass, strand that has a central core for carrying light and a surrounding cladding that facilitates internal reflection of light back into the core. During use, light is transmitted into the core of one end of the optical fiber at an acceptable angle. The incident light then travels down the fiber core to the other end of the fiber.
A typical single mode optical fiber has a core diameter of approximately 9 microns while a multi-mode fiber has a core diameter of approximately 50 or 62.5 microns. Because of the small dimensions of optical fiber cores, aligning optical fibers with laser transmitting and receiving devices, which have aperture sizes that vary from approximately 2 to 10 microns, is difficult. The problem is particularly acute when devices are aligned with single mode fibers because of their small core diameter.
There are two techniques for aligning optical fibers and devices on a package or platform: passive alignment and active alignment. Active alignment is performed by activating a light emitting device, bringing the optical fiber into position for mounting on the package, and selecting the location for mounting when the amount of light being coupled into the optical fiber (or photo-detector of the device) exceeds a given threshold. By contrast, passive alignment is performed based on the geometry of the components for assembly, without active use of a laser in the alignment process. Accordingly, passive alignment relies on placement accuracy and acceptable manufacturing tolerances to produce a reliable and repeatable assembly process.
Passive alignment based on a single integrated optical platform has been somewhat successful for multi-mode fiber coupling. This is because of the relatively large core diameter of multi-mode fibers. However, improved alignment techniques are required. In contrast, passive alignment of single mode fibers has been elusive. This is because of the tight tolerances and limitations required to optically couple a laser beam between the aperture of a laser transmitting or receiving device and the small core of a single mode fiber. The problem is exacerbated by the use of inexpensive though desirable materials for the platform, which may inherently have manufacturing tolerances that are difficult to control.
A typical method for mounting semiconductor devices onto optical platforms uses flip-chip bonding techniques, which are adapted from well-established technology from the electronics industry. Without modification, the tolerance offered by flip chip bonding of approximately 1 micron does not meet the required tolerance of 0.5 microns required for coupling an optical beam from a mounted device into a single-mode fiber.
Conventionally, a silicon platform (or package) has been used to mount a laser transmitting or receiving device and optical fibers. The platform has included a flat, slanted reflective surface of silicon that directs a laser beam between the device and a fiber. The fiber has been anchored on a v-shaped groove etched into the same silicon substrate so that the entire package is compact. This design is known as the silicon optical bench. Recently, a v-groove has been used for fiber array attachment with a vertical cavity surface emitting laser (VCSEL) device attached on a separate quartz plate.
The flat, slanted reflecting surface of these designs has several fundamental drawbacks, including: (i) the limited working distance defined as the distance between the device and the optical fiber; (ii) the failure to compensate the beam profile in the case of diode laser coupling; and (iii) high aggregate placement tolerances between the opto-electronic device and the fiber.
In order to achieve high coupling efficiency, the working distance between the opto-electronics device and the optical fiber should be kept to a minimum commensurate with geometry and other practical construction considerations. For instance, the optimum distance can be achieved by direct butt coupling of the fiber with the active region of the semiconductor device resting on a particular platform. However, this coupling scheme may not always be possible, as it is often limited by physical constraints. These issues greatly reduce or prohibit the use of this platform for the single-mode or multi-mode laser packaging using semiconductor diode lasers. Where the laser and fiber are mounted on separate platforms, active alignment is required.
Silicon v-grooves for receiving optical fibers have been conventionally formed by chemically etching precise shapes in the crystalline structure of silicon. However, single crystal silicon is very expensive, compared to glass and plastics. Polymer molding technology has been used to design a waveguide having a lens system, a 45 degree reflection prism and ferrule bore. Other waveguides have used plastic injection molding to produce a complex optical multiplexer integrated with a filter block, a 45 degree reflection prism, a lens system and fiber ferrule core. However, these waveguides have not conventionally provided a sitting platform for a laser transmitting or receiving device. Thus, active alignment has been needed to place and align the separately packaged device with the waveguide, which is labor intensive and costly. Still other modular platforms have been deployed that have several pieces requiring active alignment.
Unlike the batch packaging processes that are efficiently used for some electronics chips, the packaging of opto-electronics components using active alignment techniques has to be done one-by-one. This adds to the cost of the assembly. To a lesser degree, the handling of many small mechanical components of hundreds of micrometers to a few millimeters in size and the sealing in a hermetic environment also adds to the cost of packaging.
High cost has never been a major issue in long haul communications as these components are used and shared by many users. However, the emergence of the short reach metro/access markets, which is the next growth area in photonics, increases the need for low-cost packaging because these markets are extremely price-sensitive. The high-volume, low-cost demand must be met with significant improvement in manufacturing economics. The way to lower packaging cost is the use of more integration of photonic components in a single platform, passive alignment techniques, batch manufacturing, and the introduction of more automation in manufacturing.
Accordingly, there is a need for a new technique for mounting and aligning light transmitting and receiving devices with optical fibers that permits low-cost passive alignment techniques to be used. There is a further need for a single, integrated platform to be used for mounting laser transmitting and receiving devices and optical fibers. There is still a further need for a platform and mounting method that uses techniques to relax manufacturing tolerances and that allows the use of inexpensive materials for the mounting platform, such as plastic or glass.